what is $.expr[":"] in javascript or jQuery?

what is $.expr[":"] in javascript or jQuery?

here is the link to original code:
http://css-tricks.com/snippets/jquery/make-jquery-contains-case-insensitive/#comment-518214
http://css-tricks.com/snippets/jquery/make-jquery-contains-case-insensitive/#comment-518214
1) in th above code, what is $.expr[:]? 2) what is $.expr.createPseudo?
I can't find any document about it!!! Why they provide this without
document? very frustrated!!!!!!!!!!!